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Published September 1, 1983 | Published
Journal Article Open

Mitogenic hormones and tumor promoters greatly increase the incidence of colony-forming cells bearing amplified dihydrofolate reductase genes


Previous work has shown that the presence of a phorbol ester tumor promoter, phorbol 12-myristate 13-acetate (PMA), during a single-step selection for methotrexate (MTX)-resistant mouse 3T6 cells results in an up to 100-fold increase in the incidence of MTX-resistant, colony-forming cells. MTX resistance of most of these cells is due to amplification of the gene for dihydrofolate reductase (DHFR), the target enzyme for MTX. We show here that other active, noncytotoxic phorbol ester tumor promoters, such as phorbol 12, 13-didecanoate and 20-phorbol 12, 13-butyrate, at their optimal concentrations (approximate to 0.1 µM) are approximately equal to PMA in increasing the incidence of MTX-resistant 3T6 colonies. Mezerein, a potent second-stage tumor promoter, but a weak complete promoter, increases the incidence of MTX resistance up to 350-fold, the strongest effect for any of the agents so far tested. PMA analogs that are inactive as tumor promoters, such as phorbol or phorbol 12, 13, 20-triacetate, have no effect on the incidence of MTX-resistant 3T6 colonies. Anthralin, a nonphorbol tumor promoter, is approximate to 40% as active as PMA in the MTX selection assay. Remarkably, the hormones insulin, arginine vasopressin, and epidermal growth factor, all of which are mitogenic for 3T6 cells, also exert a strong PMA-like effect on the incidence of MTX-resistant 3T6 colonies under conditions of MTX selection. The effect of insulin at its optimal concentration (approximate to 1 µg/ml) is approximate to 70% that of PMA. Although the effect of PMA on the incidence of MTX-resistant 3T6 colonies does not significantly depend on the initial density of seeded cells or volume of the medium added, the analogous effect of insulin is strongly influenced by these parameters. Mevalonic acid, arachidonic acid, thymidine, caffeine, and nicotine, all of which are known to influence patterns of DNA synthesis in mammalian cells, were tested at their highest noncytotoxic concentrations and failed to produce any significant effect on the incidence of MTX-resistant 3T6 colonies. We discuss possible mechanisms of hormone- and tumor promoter-facilitated gene amplification in mammalian cells, relationship of mitogenic hormones to tumor promoters, and also implications of our findings for the problem of drug resistance in cancer chemotherapy.

Additional Information

© 1983 by the National Academy of Sciences. Communicated by Gerald N. Wogan, May 23, 1983. We are greatly indebted to Robert Schimke for the pDHFR11 cDNA clone. We also thank Igor Roninson and Robert Snapka for their comments on the manuscript. This work was supported by grants to A.V. from the National Cancer Institute (CA30367 and CA33297). J.B. was supported by a departmental training grant from the National Institutes of Health. The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S.C. §1734 solely to indicate this fact.

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